High-tech, high-capacity batteries are becoming an increasingly popular solution for managing the variability of renewable generation, one of the major challenges of integrating large amounts of renewable energy into North America’s power grids.
While managing the variability of wind and, to a lesser extent, solar resources has been a challenge for control room operators as long as renewable energy has been contributing power to the grid, the amount of energy from such variable energy resources (VERs) has been steadily increasing. The higher the percentage of renewable energy in the mix, the greater the challenge of managing the variability.
When fewer VERs were in the generation mix, utilities used assets already at their disposal to manage the inherent variability. Even today, resources used for balancing variability often include balancing area authority reserves, many of which rely upon reliability-must-run spinning generators that are otherwise in place to support bus voltages within selected load pockets, Tom Wray, project manager for the SunZia Southwest transmission project, told TransmissionHub.
However, with the amount of renewable energy poised to increase in the years ahead, in part to meet renewable portfolio targets or mandates, the burden on utilities and balancing authorities will also increase. The anticipation of the additional burden to come may already be contributing to changes in who is responsible for firming those variable resources.
“More and more, responsibility will be shifted to wind producers to deliver electricity that has already been firmed,” a spokesperson for A123 Energy Solutions, a manufacturer of utility-scale storage batteries, told TransmissionHub.
In early June, A123 Energy installed and placed into service its second battery system on the Hawaiian island of Maui, and connected the system to Maui Electric’s grid. While primarily intended to increase electric grid operational efficiency, stability, and power quality, the storage system will also provide wind curtailment relief, in addition to other benefits, the company said.
The company’s products include a long-duration battery system that can deliver 4 MW of energy for a full hour. Other systems provide the energy for high power demand applications for approximately 15 minutes. Even the shorter of the two timeframes can provide a buffer sufficient for control room operators or the balancing authority to dispatch quick-start generators, which often have start-up times in the 10-minute range, when the variable energy generation subsides and the batteries take over.
“It’s a really cost-effective way to provide that predictability, using batteries and wind together,” the A123 Energy spokesperson said.
Another company that has successfully incorporated utility-scale batteries to manage variability is AES Energy Storage, a subsidiary of AES Corp. (NYSE:AES). In September 2011, the company opened its first energy storage project at a wind farm in West Virginia, in the PJM Interconnection’s (PJM) territory.
“AES has a [32 MW] battery array at its Laurel Mountain 98 MW wind farm,” a spokesperson for PJM told TransmissionHub. “It uses the batteries to balance and ramp output from the farm and provide regulation service.” The sealed lithium-ion batteries, originally designed for buses, fill rows of 53-foot long shipping containers.
The buffer time of even 15 minutes can be sufficient to avert incidents such as those that occurred in ERCOT in 2008 when wind production fell from more than 1,700 MW to 300 MW. Even that margin would allow grid operators to ramp up generators with spinning reserves and start up those quick-start assets that are not presently online.
Location, location, location
While many producers are positioning batteries for wind energy storage at a central location on the wind farm or near substations, other manufacturers of VERs are approaching the use of storage to manage renewable variability in a more granular manner. For example, wind turbine manufacturer General Electric (NYSE:GE) in May announced its first project to integrate battery energy storage at the base of individual wind turbines.
“One of the [challenges of renewable energy] is around predictable power, and what our battery is able to do is forecast the power that is put onto the grid [in increments] from 15 minutes to 60 minutes,” a spokesperson for GE Renewables told TransmissionHub. “This provides the certainty and predictability of knowing what wind power will be put onto the grid.”
“Instead of a trailer of batteries at the wind-farm level, we’re combining a little bit of storage” at each turbine, and coupling that with advanced wind forecasting to optimize firm delivery, the spokesperson said.
For his part, Wray believes simple-cycle gas turbines located near the source of the renewable generation will be the preferred method for firming renewables.
In fact, many balancing authority areas already rely on such quick-start assets to balance renewable variability. Those assets may be owned and operated by the renewable energy generators themselves, but are more likely to be owned by other operators who make the units’ capacity available either through bilateral agreements, or through the real-time and/or ancillary services markets in those areas where such organized markets exist.
Regardless of which solution or combination of solutions ultimately prevails, Wray agrees there is a change in the works with regard to the supplying of renewable energy.
“The need for regulation and supply firming is ultimately determined in the bilateral power purchase agreement terms that are settled between the selling renewable generator and purchasing electric utility,” Wray said. “Future interstate transmission lines like SunZia that are largely conceived to access stranded high capacity-factor renewable energy resources, will by necessity find themselves more involved in generator-utility customer arrangements to assist with this regulation, balancing and firming requirement.”